Photocatalytic Pollutant Oxidation

Photoactivated titanium dioxide will also catalyse the oxidative degradation of organic material in solution. Such photocatalytic oxidation reactions are applied very successfully in treatment and removal of organic pollutants from water and from air. Solar photocatal) ic degradation of pollutants or bacteria in water could also provide a cheap and facile route to clean water in parts of the world where traditional water-purifying infrastructure is not available. 

As well as water purification, there are many other areas where photo-catalytic contaminant oxidations can he applied such as self-cleaning windows and surfaces and air purification. 

Titanium dioxide (hut more importantly titanium-containing materials with more dispersed titanium atoms) shows potential for use in the fixation of carbon dioxide via photocatalytic conversion of carbon dioxide and water into carbon monoxide, formic acid, methanol, and methane. Such an artificial photosynthesis of carbon-containing compounds would obviously be extremely valuable both in the production of solar fuels and the amelioration of the effects of carbon dioxide in global warming and it can be expected that these efforts will continue to grow. Again the process is initiated by UV light to promote an electron from the VB to the CB. The hole in the VB oxidises water to form protons and 0) gen and the electron in the CB reduces carbon dioxide and protons to form a variety of industrially relevant compounds. 

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It is well known that photocatalytic oxidation of or nic pollutants follows Langmuir-Hinshelwood kinetics[6]. Therefore, this kind of reaction can be represented as follows. 

The oxidative degradation of organic pollutants in water and air streams is considered as one of the so-called advanced oxidation processes. Photocatalytic decomposition of organics found widespread industrial interest for air purification (e.g., decomposition of aldehydes, removal of NO , ), deodorization, sterilization, and disinfection. Domestic applications based on Ti02 photocatalysts such as window self-cleaning, bathroom paints that work under illumination with room light, or filters for air conditioners operating under UV lamp illumination have already been commercialized. Literature-based information on the multidisciplinary field of photocatalytic anti-pollutant systems can be found in a number of publications, such as Bahnemann s [237, 238] (and references therein). 

Photocatalytic oxidation can be an effective way of removing pollutants in the gas phase, such as NOx, SOx, and volatile organic compounds (VOCs). 

This section covers environmental applications of nanomaterials insofar as they are directly applied to the pollutant of interest. The photocatalytic degradation of organic pollutants and remediation of polluted soils and water are discussed here. The high surface areas and photocatalytic activities of semiconductor nanomaterials have attracted many researchers. Semiconductor nanomaterials are commercially available, stable, and relatively nontoxic and cheap. Prominent examples that are discussed are metal oxides such as Ti02 and ZnO and a variety of Fe-based nanomaterials. 

Several articles have reviewed the ongoing work in the photocatalytic degradation of pollutants that involve oxidation or reduction processes (depending on the experimental conditions) [16,18,187,265-273], The addition of external oxidants such as ozone or hydrogen peroxide during the photocatalytic process can improve the degradation of the organic material when they are added in suitable doses [274-275],... 

Byrne JA, Eggins BR, Byers W, Brown NMD (1999) Photoelectrochemical cell for the combined photocatalytic oxidation of organic pollutants and the recovery of metals from waste waters. Appl Catal B Environ 20 L85-89... 

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

Such high photocatalytic reactivities of photo-formed e and h can be expected to induce various catalytic reactions to remove toxic compounds and can actually be applied for the reduction or elimination of polluted compounds in air such as NO cigarette smoke, as well as volatile compounds arising from various construction materials, oxidizing them into CO2. In water, such toxins as chloroalkenes, specifically trichloroethylene and tetrachloroethene, as well as dioxins can be completely degraded into CO2 and H2O. Such highly photocatalyti-... 

Fig. 8.2 Principle of photocatalytic oxidation of various air pollutants under UV light illumination.

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